Promoting technical people with no management experience into management jobs, without providing them with any training or guidance. (Happened to me.) Writing code and managing people require very different sets of skills, and just because you're good at the former doesn't necessarily mean you'll be any good at the latter (or that you'll enjoy doing it).
(Similar problems can happen when a bunch of people with no management skills decide to found a company and start hiring people.)
(Similar problems can happen when a bunch of people with no management skills decide to found a company and start hiring people.)